For instance, water vapor in the atmosphere isindependent of the presence of other gases. The vaporpressure is independent of the pressure of the dry gasesin the atmosphere and vice versa. However, the totalatmospheric pressure is found by adding all thepressuresthose of the dry air and the water vapor.TERMSThe actual amount of water vapor contained in theair is usually less than the saturation amount. Theamount of water vapor in the air is expressed in severaldifferent methods. Some of these principal methods aredescribed in the following portion of this section.Relative HumidityAlthough the major portion of the atmosphere isnot saturated, for weather analysis it is desirable to beable to say how near it is to being saturated. Thisrelationship is expressed as relative humidity. Therelative humidity of a volume of air is the ratio (inpercent) between the water vapor actually present andthe water vapor necessary for saturation at a giventemperature. When the air contains all of the watervapor possible for it to hold at its temperature, therelative humidity is 100 percent (See fig. 1-12).Arelative humidity of 50 percent indicates that the aircontains half of the water vapor that it is capable ofholding at its temperature.Relative humidity is also defined as the ratio(expressed in percent) of the observed vapor pressure tothat required for saturation at the same temperature andpressure.Relative humidity shows the degree of saturation,but it gives no clue to the actual amount of water vaporin the air. Thus, other expressions of humidity areuseful.Absolute HumidityThe mass of water vapor present per unit volume ofspace, usually expressed in grams per cubic meter, isknown as absolute humidity. It may be thought of as thedensity of the water vapor.Specific HumidityHumidity may be expressed as the mass of watervapor contained in a unit mass of air (dry air plus thewater vapor). It can also be expressed as the ratio of thedensity of the water vapor to the density of the air(mixture of dry air and water vapor). This is called thespecific humidity and is expressed in grams per gram orin grams per kilogram. This value depends upon themeasurement of mass, and mass does not change withtemperature and pressure. The specific humidity of aparcel of air remains constant unless water vapor isadded to or taken from the parcel. For this reason, airthat is unsaturated may move from place to place orfrom level to level, and its specific humidity remainsthe same as long as no water vapor is added or removed.However, if the air is saturated and cooled, some of thewater vapor must condense; consequently, the specifichumidity (which reflects only the water vapor)decreases. If saturated air is heated; its specifichumidity remains unchanged unless water vapor isadded to it. In this case the specific humidity increases.The maximum specific humidity that a parcel can haveoccurs at saturation and depends upon both thetemperature and the pressure. Since warm air can holdmore water vapor than cold air at constant pressure, thesaturation specific humidity at high temperatures is1-20THE DIFFERENCE BETWEENACTUAL TEMP. AND DEW POINTTEMP. IS AN INDICATION OFHOW CLOSE THE AIR IS TOSATURATION.IF COOLED TO DEW POINTTEMP. OR ADDITIONALWATER VAPOR IS ADDEDTO SATURATED AIR,CONDENSATION OCCURS.DRY AIRWATER VAPORAIR TEMPDEW POINTRELATIVE HUMIDITY4060808080808040100100100100606050OOOOOOOOOOOO%%%%%FFAGf0112Figure 1-12.Relative humidity and dew point.